The purpose of these studies is to investigate the molecular mechanisms of determination and of differentiation during erythropoiesis and myogenesis. Our studies will be concerned with possible changes in the chick embryonic genome during determination. We want to study the proposition that determination is due to somatic recombination, which causes related gene sequences to join and thereby become active and expressed. Alternatively, a recombination event might cause gene sequences to be relocated during determination to a region of the genome where they can be expressed. Secondly and more concerned with the somewhat later events of differentiation, the processing of mRNA sequences by a splicing enzyme has become a focus of interest in the last two years. We want to investigate the role of processing by RNA splicing in the differentiation of the early chick embryo.